Hot melt adhesive compositions are well known in the art. Such hot melt adhesives are typically solid at room temperature and are applied to a substrate when molten, and then placed in contact with a second substrate. The adhesive cools and hardens to form a bond between the substrates. Hot melt adhesives are typically composed of at least two components: (a) a polymer; and (b) a resin. Optionally, various amounts of wax and plasticizer may further be employed. The polymer provides the formulation with its strength and adhesive characteristics. Typically, a thermoplastic polymer is used. The thermoplastic polymer may be selected from homopolymers, copolymers (e.g. interpolymers) or block copolymers. The resin allows the thermoplastic polymer to be more adhesive by improving wetting during the application. Resins are added to give tack to the adhesive and also to lower viscosity. Tack is required in most adhesive formulations to allow for proper joining of articles prior to the hot melt adhesive solidifying. One function of the wax component is to lower the viscosity of the hot melt adhesive. Low viscosity helps to lower application temperatures, provide cleaner processing, and also good wet out of the substrates. Furthermore, a low viscosity allows for liquefaction of the hot melt adhesive. In addition, due to the presence of the wax component, the hot melt adhesive crystallizes quickly which helps the material to harden or set quickly. Thus, the wax component also controls the open time and set speed of the system. The plasticizer also helps to lower viscosity and can additionally be used to impart permanent tack properties to the hot melt adhesive.
It is known in the art to use hot melt adhesives for bonding containers into packs. For example, WO 2013/004340 and WO 2013/004337 describe methods for producing packs of containers by bonding the containers to each other using an adhesive such as a hot melt adhesive. By directly bonding containers such as cans using an adhesive, waste resulting from a shrink wrap with LDPE films on the packs can be avoided. This also saves considerable amounts of energy since, for example, shrink wrapping involves six-packs being moved through a heating tunnel which uses large amounts of energy. Furthermore, a smaller packing line footprint and increased packing line speeds are possible. Also, the optical appearance of six-packs is improved, since undesirable creases in the shrink wrap are avoided. This is highly desirable, because beverage filling companies are especially concerned about the appearance of their packaged product. Generally, any secondary packaging of container packs, such as carton or cardboard cases or sheaths, plastic rings or sheaths, etc. can be limited or even avoided.
However, when using an adhesive such as a hot melt adhesive for directly bonding containers to each other, it is necessary to provide on the one hand a reliable bond between the containers. On the other hand, it is also required to ensure easy separation of the containers so that an individual container can be separated from the pack. Generally, a reliable bond can be provided by using a higher adhesive force, i.e. a stronger bond. By contrast, easy separation can be ensured by applying a lower adhesive force. Thus, providing simultaneously for a reliable bond and for easy separation represent conflicting goals.
Containers can be made from a variety of materials. Therefore, a hot melt adhesive composition used for bonding containers should be specifically selected to reliably bond a given material. Metals such as aluminum or steel are widely used for making containers such as cans. Thus, there is a need in the art for hot melt adhesives for reliably bonding metal containers to each other into packs, while at the same time being easily separable from each other. In particular, the hot melt adhesive at the same time should be able to bond metal cans as well as coated metal cans, e.g. varnish coated, printed or plastic coated cans, etc. It is further desirable that such hot melt adhesives are removable (i.e. can be stripped cleanly from the container, leaving no adhesive residue behind) after use, e.g. during recycling.